This invention relates to air filters and methods of manufacturing the same. In particular, the invention concerns corrugated paper filter structures using activated carbon for cleaning air.
Such active carbon filters are employed in air cleaning equipment such as room air cleaners and other air filtration devices in which, for example, air is passed over the filter by means of an air moving device such as a fan whereby the active carbon filter removes, inter alia, organic gases, radon, ozone and NO2 from the air-flow through adsorption.
The performance of the filter is determined by the gas-removal efficiency, the capacity and the pressure drop (air resistance). The gas-removal efficiency is determined, along with the absorption capacity, by the quantity of effective active carbon, and, along with the pressure drop, also by the structure of the filter.
An object of the present invention is to provide an activated carbon air filter which has improved performance.
It is another object of the present invention to provide an activated carbon air filter which provides a high gas-removal efficiency and a high capacity combined with a low pressure drop and a small volume.
The invention provides a carbon filter of corrugated paper structure which is optimised in terms of its cost, the obtained carbon weight per unit filter volume, the corrugation specifics of the filter, the face area of the filter and the filter thickness, the pressure drop across the carbon filter, the adhesion of the carbon coating to the corrugated paper support medium, and which can be utilised advantageously in an air cleaner wherein specific boundary conditions exist with respect to the process conditions, particularly air speed, pressure drop, filter volume, and one-pass air cleaning performance.
According to one aspect of the present invention there is provided an activated carbon air filter comprising a corrugated paper structure carrying activated carbon, which is characterised in that the paper comprises a substantially non-porous, organic fiber paper and in that the activated carbon is provided as a coating on the paper surface.
According to another aspect of the present invention there is provided a method of making an activated carbon air filter comprising a corrugated paper structure carrying activated carbon, which method comprises forming the corrugated paper structure from substantially non-porous, organic fiber paper and thereafter applying the activated carbon in the form of a coating on the surface of the corrugated paper.
Conventionally, activated carbon paper air filters comprise an inorganic fiber, porous, paper which is impregnated with the carbon, with the carbon particles dispersed in the voids between the paper fibers, prior to the paper being corrugated and formed into a honeycomb structure. Such filters tend to have a limited carbon content, and typically the carbon loading is around 70 gms/liter of filter volume.
Through using a substantially non-porous paper and providing the carbon as a coating on the surface of the paper, with the carbon coating confined and adhering to the paper surface, considerable improvement in the operational effectiveness of the filter is obtained. In a preferred embodiment, the corrugated paper is formed into a honeycomb structure having air channels approximately 1 mm in diameter, preferably using an E-flute construction having a cell height between 1.2 to 1.4 mm and a pitch of substantially 3 mm, and with a carbon coating approximately 0.1 to 0.2 mm thick, then a carbon loading of approximately 130 gm of carbon per liter of filter volume is achieved which dramatically improves air cleaning performance. The channel size selection results in filter functionality, particularly efficiency, lifetime, pressure drop and filter volume, being optimised.
Preferably, the non-porous paper comprises Kraft paper, desirably having a density of approximately 49 gm/m2. The fiber density of such paper is sufficient to prevent carbon particles of around 50 xcexcm or more diameter penetrating the fibrous interior, thus ensuring that the carbon remains confined to the paper surface to which it adheres. Kraft paper offers the advantages of low cost, ease of paper corrugation before coating, and low weight as well as providing desired properties for ensuring adequate adherence of the carbon coating to its surface.
In a preferred method of making the air filter, the carbon coating is applied using an aqueous slurry; including a binder, preferably carboxymethylcellulose, which provides consistency and promotes good adhesion of the coating, to the paper. The coating is applied by partly immersing the corrugated paper structure in the slurry, preferably with prior wetting of the paper, and drawing the slurry up through the corrugation channels, for example using capillary action or by sucking, and thereafter forcing the slurry out, by air blowing or the like, and drying, thereby leaving a homogeneous and dense carbon coating on the paper surface.
According to a further aspect of the present invention, there is provided an air cleaning device comprising an activated carbon air filter in accordance with the first aspect of the invention and air moving means operable to produce a flow of air through the air filter.